The present invention relates to a method and apparatus used to coin a valve seat in a fuel injector to improve seating between the valve seat and a needle in the injector.
The metal-to-metal seal formed in a valve between a needle and a seat determines the accuracy at which the fluid flowing through the valve is controlled. Leakage results when the surfaces between the needle and the seat do not mate correctly. This leakage, no matter how small, is detrimental in systems where precise flow control is desired.
One of the uses of a fluid valve of the type to which the present invention relates is as a fuel injector for injecting a combustible fuel into a combustion engine. In the case of a spark-ignited, internal combustion engine for an automotive vehicle, the valve is typically under the control of an electronic control system and injects gasoline into the engine.
One of the chief reasons for using a fuel-injected engine is because of the ability to closely control the amount of fuel entering the engine. Close control over injected fuel is important for reasons of both fuel economy and exhaust emissions. When a fuel injector is closed, it should totally prevent fuel from leaking into the engine because such leakage can have undesired consequences. For example, even small amounts of leakage can adversely affect exhaust emissions in very significant ways. Certain countries now impose limits on the amounts of certain exhaust constituents that can be discharged to the atmosphere, and there is a trend toward making these limits even stricter. Accordingly, a commercially acceptable fuel injector is likely to have to comply with increasingly stringent limits on leakage.
The method and apparatus used to manufacture the needle and the seat greatly influence the accuracy and reliability of the fluid valve. Extremely costly manufacturing procedures could, of course, be invoked to ensure precise surface finishes and fits of the cooperating needle and seat by placing extremely small tolerances on the dimensions and surface finishes of the parts involved. Such activities would obviously increase the manufacturing costs, possibly to non-competitive prices for some companies. Alternate procedures that are less costly are therefore desirable.
One known method for surface finishing the needle and seat involves a grinding process. The mating surfaces of the needle and the seat are pressed into contact. Then, either the needle or the seat is rotated relative to the other. This grinding of the mating surfaces of the needle and the seat is performed in the presence of a slurry of fine-grained lapping medium. Vibrating the needle and the seat in the axial direction of the needle valve further complicates this known process. The vibration of these two valve elements is performed at the same frequency but at a different amplitude to impart a pumping action on the slurry.
Another method for manufacturing the needle and seat applies an axial compressive load to force the needle against the seat, coining the needle to the seat. The method described in U.S. Pat. No. 5,081,766 produces a valve that is capable of accurate and reliable fluid metering yet avoids expensive tolerance control on surface finishing and part dimensioning. The method disclosed by this patent involves the inclusion of an additional step in the manufacturing process, the coining step, but eliminates the necessity for stricter tolerances on surface finish and part dimensions. Accordingly, reconfiguration of existing manufacturing equipment and processes requires merely adding the coining step to reduce leakage through the injector. This coining step however does not involve the use of a coining die to coin a part. Rather, the coining step involves the application of axial compressive load to force a rounded distal end of the needle against a frusto-conical surface of the seat so that coining action occurs at an annular zone of surface contact between the needle and the seat. The force application is preferably conducted in a particular manner so that the needle is neither irreversibly bent nor buckled by the coining step. This step is conducted during the manufacturing process so that neither the solenoid nor the spring, which are parts of the operating mechanism in the completed injector, has an influence on the result of the coining.
Known manufacturing equipment typically comprises a fluid powered piston device to apply the axial compressive load. However, the compressive load is applied only one time during the manufacturing of the injector. If the needle and seat are laterally or rotationally displaced from one another after coining, the coining effect may be lost. It would be beneficial to develop a method of applying the compressive load multiple times during the manufacturing of the injector to form a better seal between the needle and the seat.
Briefly, a method for making a valve assembly is provided. The method comprises providing a first work piece having a longitudinal axis and a first end and providing a second work piece having a surface. The method also comprises the first work piece and the second work piece axially between a pair of aligned elements that are relatively movable toward and away from each other along the longitudinal axis of the first work piece. Additionally, the method comprises relatively moving the pair of aligned elements toward each other to axially clamp the first work piece and the second work piece and actuating the pair of aligned elements and delivering to the first work piece and the second work piece a controlled clamping force that acts to coin a zone of surface contact between the first end and the surface. Further, the method comprises repeating the actuating of the pair of elements a plurality of times.
Additionally, the present invention provides a coining apparatus. The coining apparatus comprises a frame, a first clamp having a first work piece receiving portion, and a second clamp axially aligned with and opposing the first clamp. The second clamp has a second work piece receiving portion. One of the first clamp and the second clamp is connected to the frame. The coining apparatus also comprises a vibrator attached to the frame, distal from the one of the first clamp and the second clamp connected to the frame and a cylinder connecting the vibrator and the other of the first clamp and the second clamp. The cylinder includes a rod reciprocally extending therefrom. The rod is connected to one of the vibrator and the other of the first clamp and the second clamp.